Optical rotary encoder with indexing

ABSTRACT

An optical rotary encoder having a rotatable pulse scale having a row of code pattern with specific light-permeable or-deflection slits each for outputting a reference position signal and other light-permeable or reflection slits each adapted to reduce the amount of light for outputting an angular signal, both of the optical signals being photoelectronically converted and the waveform of the resultant electric signals being shaped based on threshold voltages into electric pulse signals indicative of a reference angle of 0° and the accumulated angle of rotation, in which the circumferential width of each specific reference slit is made narrower than that of other slit so that the pulse width for angular position indicating of the angular pulse signal is always equal and constant for ensuring more accurate detection of the angle of rotation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention concerns an optical rotary encoder for detecting the angle of rotation of a rotatable pulse scale based on an angular signal in accordance with the angle of rotation and a reference position signal indicative of 0° for the angle.

2. Description of the Prior Art

An optical rotary encoder has been used as a means for detecting the crank angle of an automobile engine, for example, in a case of controlling the ignition timing to a spark plug or in a case of varying the ignition timing in accordance with the number of rotations of the engine.

The optical rotary encoder of the prior art, as shown in FIG. 4 of the appended drawings, has a code pattern C₁ for detecting an angle of rotation and a code pattern C₂ for detecting a reference position formed in two concentric rows on a rotatable pulse scale 1, such that two kinds of optical pulse signals are respectively outputted.

Light is projected from two light emitting devices 51 and 52 by way of two light emitting optical fibers 53 and 54 into the code patterns C₁ and C₂, respectively. Then, optical pulse signals outputted from the patterns are sent by way of two light receiving optical fibers 55 and 56 and then optoelectronically converted through respective photoreceiving devices 57 and 58, thereby causing these devices to detect angular signal S₁ and a reference position signal S₂.

For instance, in a 6 -cylinder engine, a code pattern C₂ is formed for judging respective cylinders such that the reference position signal S₂ is outputted on every 60° rotation of the pulse scale 1, and the timing for ignition is controlled depending on the number of pulses contained in the angular signal S₁ after the input of the pulse of the reference position signal S₂.

The rotational shaft of the pulse scale 1 is connected with the crank shaft of an engine, while a control device 59 equipped with electronic parts sensitive to heat or vibrations such as light emitting devices, 51, 52 and photoreceiving devices 57, 58 are disposed within the dashboard, in front of the driver's seat, remote from the engine, so that these devices are protected against direct effects of heat or vibrations from the engine.

However, if the light emitting devices 51, 52, and the photoreceiving devices 57 and 58 are disposed each in pairs since the light emitting optical fibers 53, 54 and the light receiving optical fibers 55, 56 have to be connected by means of respective optical connectors which are each of a great diameter, it is difficult to accommodate the control device 59 in the narrow space behind an automotive dashboard.

Since the optical pulse signals outputted from a rotary encoder consist only of "1" and "0" signals, it is required to form two rows of code patterns on the pulse scale 1 and, correspondingly, two series of optical systems in order to obtain two kinds of information, that is, the angular signal and the reference position signal. This requires that the light emitting devices 51, 52 and the photoreceiving devices 57, 58 be provided in twos, making it impossible to reduce the size of the connection portion of the device.

In addition, as the number of devices is increased, the number of relevant parts is also increased correspondingly, making the assembling work troublesome, as well as increasing the cost.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide an optical encoder capable of obtaining two kinds of information of an angular signal and a reference position signal, by merely disposing a row of code pattern on a pulse scale, thereby reducing the number of devices and decreasing the cost, as well as reducing the size of the connection portion between optical fibers and a control device.

In the first aspect of the present invention, the foregoing object can be attained by an optical rotary encoder in which an angular signal and a reference position signal indicative of 0° for the angle are outputted as optical pulse signals in accordance with the angle of rotation of a pulse scale and the angle of rotation of the pulse scale is detected based on the angular signal and the reference position signal, wherein the optical rotary encoder comprises:

a rotatable pulse scale having a row of code pattern formed along the circumferential direction thereof and comprising a specific light permeable or light reflection portion for outputting optical signals at a higher peak level containing the optical angular signal and the optical reference position signal together and a light permeable or light reflection portion adapted to reduce the amount of transmission light or reflection light for outputting optical signals at a lower peak level containing only the optical angular signal, and

a signal processing section for discriminating binary "1" or "0" for electric pusle signals obtained by optoelectronic conversion of the optical signals and waveform shaping of the resultant electric signals based on a first threshold voltage for the angular signal which is set lower than the lower peak level and a second threshold voltage for the reference position signal which is set between the higher peak level and the lower peak level, and wherein

the circumferential width of the specific light permeable or light reflection portion is made narrower than that of other light permeable or light reflection portion in the code pattern of the pulse scale, so that the width of the electric signal pulse at the higher peak level obtained by the waveform shaping is made equal with the width of the electric signal pulse at the lower peak level obtained by the waveform shaping, each based on the first threshold voltage.

In the present invention, a code pattern formed along the circumferential direction of a pulse scale comprises a specific light permeable or light reflection portion and a light permeable or light reflection portion formed so as to reduce the amount of transmission light or reflection light. Accordingly, optical signals comprising an angular signal and a reference position signal superposed thereon is outputted as an optical signal at a higher peak level from the specific light permeable or light reflection portion, while optical signals at a lower peak level are outputted as an angular signal from other light permeable or light reflection portion.

Then, the optical signals containing two kinds of information for the angular signal and the reference position signal present together are optoelectronically converted and the resultant electric signal is shaped into a pulse waveform by comparison with first and second threshold (reference) voltages.

Since the first threshold voltage for the angular signal is set to a predetermined value which is lower than the lower peak level, all of the electric pulses are detected as the angular signal.

On the other hand, since the second threshold voltage for the reference position signal is set between the higher peak level and the lower peak level, only the pulse at the higher peak level outputted from the specific light permeable or light reflection portion in the code pattern of the pulse scale is detected as the reference position signal.

That is, by projecting the light outputted from a light emitting device onto one code pattern of the pulse scale, a series of optical pulse signals containing two kinds of information comprising a portion at a higher peak level and another portion at a lower peak level is outputted, and the signal detected by one photoreceiving device can be separated into the angular signal and the reference position signal.

Accordingly, the required number of the light emitting devices and the photoreceiving devices can be reduced to one of each to decrease the number of parts and, accordingly, to reduce the labor needed for the assembling operation, and thereby decreased the cost. In addition, the size of the connection portion for connecting the light emitting and light receiving optical fibers with the connection device can be reduced in size.

In particular, in the present invention, since the circumferential width of the specific light permeable or light reflection portion in the code pattern formed on the pulse scale is made narrower as compared with that of other light permeable or light reflection portions so that the width of the electric signal pulse at the higher peak level obtained by the waveform shaping is made equal with the width of the electric signal pulse at the lower peak level obtained by the waveform shaping each based on the first threshold voltage, the pulse width of the angular signal is always equal with respect to each of the portions in the code pattern and the angle of rotation can be detected at a high accuracy.

In the second and more specific aspect of the present invention, the optical rotary encoder of the above-mentioned type comprises:

(A) a rotatable pulse scale having a row of code pattern formed along the circumferential direction thereof and comprising a specific light permeable or light reflection portion for outputting optical signals at a higher peak level containing an optical angular signal and an optical reference position signal together, and a light permeable or light reflection portion adapted to reduce the amount of transmission light or reflection light for outputting optical signals at a lower peak level containing only the optical angular signal, and

(B) a control device having,

a light emitting device for projecting light onto the code pattern, and a photoreceiving device for receiving from the code pattern two kinds of the optical signals outputted which signals are superposed one upon the other, and converting them into electric signals, and also having,

a pair of threshold voltage generation means, one of these means generating a first threshold voltage which is lower than the peak level of the electric signal when only the angular signal is inputted, and the other of these threshold voltage generation means generating a second threshold voltage which is higher than the peak level of the electric signal when only the angular signal is inputted and lower than the peak level of the electric signal when both of the angular signal and the reference position signal are inputted simultaneously, and

a pair of signal processing means, one of these means comparing the electric signal outputted form the photoreceiving device with the first threshold voltage to discriminate the absence or presence of the angular signal as binary "0" or "1", and the other of these signal processing means comparing the electric signal outputted from the photoreceiving device with the second threshold voltage to discriminate the absence or presence of the reference position signal as binary "0" or "1", so that the pulse signal outputted from the photoreceiving device is separated into an angular signal and a reference position signal, and wherein

the circumferential width of one of the specific light permeable portion and light reflection portion is made narrower than that of other of the light permeable portion and light reflection portion in the code pattern, so that the width of the electric signal pulse at the higher peak level obtained by the waveform shaping is made equal with the width of the electric signal pulse at the lower peak level obtained by the waveform shaping each, based on the first threshold voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, as well as advantageous features of the present invention will become apparent by reading explanations for the preferred embodiments of the present invention in conjunction with the accompanying drawings, wherein:

FIG. 1 is an explanatory view illustrating one embodiment of an optical rotary encoder according to the present invention;

FIG. 2 is an enlarged view for a portion of a pulse scale used in the embodiment shown in FIG. 1;

FIG. 3 is a graph illustrating a relationship between the signal waveform and the pulse width; and

FIG. 4 is an explanatory view illustrating an example of an optical rotary encoder in the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described by way of its preferred embodiments with reference to the accompanying drawings.

FIG. 1 is an explanatory view illustrating the structure of one embodiment of an optical rotary encoder according to the present invention, FIG. 2 is an enlarged view for a portion of a rotatable pulse scale used in the embodiment shown in FIG. 1 and FIG. 3 is a graph illustrating a relationship between the signal waveform and the pulse width.

In this embodiment, the present invention is applied to a transmission-type optical rotary encoder, in which a row of code pattern C₀ comprising a plurality of slits 2, 2,--as a light permeable portion is formed circumferentially on a pulse scale 1. Each of the slits is formed at a predetermined circumferential distance.

When the encoder is used as a crank angle sensor, for example, in a 6-cylinder engine, each of the slits 2, 2,--is formed with an optical attenuation film for reducing the amount of transmitted light, except for specific slits 2a, 2a,--(included in the slits 2, 2,--) each formed at an angular interval of 60° for outputting a reference position signal S₂.

Specifically, the code pattern C₀ is formed, for example, by vapor-depositing a chromium metal layer on the surface of the pulse scale 1 made of a circular glass plate, to render the entire surface non transparent, except for the slit portion. The thickness of the vapor-deposited chromium layer is reduced at the portions for the slits 2, 2,--so as to form a semi-transparent optical attenuation film, while chromium metal is not deposited at all at the portions for the specific slits 2a, 2a,--to expose the transparent glass substrate as it is.

Accordingly, an optical pulse signal at a lower peak level P₁ corresponding to an angular signal S₁ is outputted through each of the slits 2, 2,--. Meanwhile, an optical pulse signal at a higher peak level P₂ corresponding to a combination of the angular signal and a reference position signal S₂ is outputted through each of the specific slits 2a, 2a,--at a predetermined period. Thus, a series of optical pulse signals containing signals of different levels are outputted together.

The circumferential width Wa of the specific slit 2a is made narrower than the circumferential width W of other slit 2. As shown in FIG. 3, the width of a signal wave outputted from the slit 2a is narrower at the bottom of its waveform than that outputted from other slit 2 and equal at the middle height of its waveform to that outputted from other slit 2.

The slits are formed such that the angle made between the center lines for adjacent slits is equal with respect to the center of the pulse scale 1 and, therefore, the peak position of the signal waveform is not deviated circumferentially, even if the width of the specific slit 2a is made narrower.

Then, when the outputted optical signal is photoelectronically converted, the electric signal thus obtained also has a similar waveform. If a reference voltage V₁ as a threshold value for the angular signal S₁ is set to such a level at which the width t_(a) of the signal waveform outputted from the specific slit 2a is equal to the width t of the signal waveform outputted from the other slit 2, the pulse width of any pulse signal indicative of the angle of rotation of the pulse scale 1 is always made equal and constant.

A control device 3 has a pair of a light-emitting device 4 and a photoreceiving device 5. A light-emitting optical fiber 6 and a light-receiving optical fiber 7 are connected each at one end thereof to the light-emitting device 4 and the photoreceiving device 5 and opposed to each other at the other ends thereof on both sides of the code pattern C₀.

The photoreceiving device 5 is connected to a signal-processing section 8 for discriminating binary "0" and "1" in the electric angular signal S₁, and a signal-processing section 9 for discriminating binary "0" and "1" in the electric reference position signal S₂.

The signal-processing sections 8 and 9 are connected respectively with reference voltage-setting circuits 8a and 9a, respectively.

The reference (threshold) voltage V₁ in the signal-processing section 8 for the angular signal S₁ is set by the reference voltage setting circuit 8a to lower than a lower peak level P₁ when only the angular signal S₁ is inputted, and a signal at a level higher than the reference voltage V₁ is discriminated to be "1".

Further, the reference (threshold) voltage V₂ in the signal-processing section 9 for the angular signal S₂ is set by the reference voltage-setting circuit 8b to lower than a lower peak level P₂ when the angular signal S₁ and the reference position signal S₂ are inputted simultaneously and higher than the peak level P₁ when only the angular signal S₁ is inputted, and a signal at a level higher than the reference voltage V₂ is discriminated to be "1".

Accordingly, as shown in FIG. 3, an optical signal waveform containing the angular signal S₁ and the reference position signal S₂ superposed thereon is inputted to and photoelectronically converted by the photoreceiving device 5. Then, the converted signal can be separated by each of the signal processing sections 8 and 9 into the angular signal S₁ and the reference position signal S₂.

In the figure, reference numeral 10 denotes a mathematical processing device for calculating the angle of rotation by counting the number of pulses in the angular signal S₁ while referring to the reference position signal S₂.

The function and the operation of the embodiment according to the present invention as shown in FIGS. 1-3, will now be described.

At first, the light emitting optical fiber 6 for projecting light-emitted from the light emitting device 4 to the code pattern C₀, and the light-receiving optical fiber 7 for guiding the optical pulse signal outputted from the code pattern C₀ to the photoreceiving device 5 are connected collectively by way of a connector (not illustrated) to the control device 3.

That is, since only one of each of the light-emitting device and the photoreceiving device 5 is necessary in this embodiment, the size of the connector for connecting the optical fibers 6 and 7 to the light-emitting device 4 and the photoreceiving device 5 can be reduced. Then, the control device 3 can be housed easily within a narrow dashboard space, and the operation for connecting the control device 3 with the optical fibers 6 and 7 can be simplified.

Further, since the code pattern C₀ for outputting the optical signal waveform comprising the angular signal S₁ and the reference position signal S₂ superimposed thereon is formed on the pulse scale 1, an optical signal wave at a lower peak level is outputted as the angular signal S₁, while a portion in which the reference position signal S₂ is superimposed is outputted from the specific slit 2a as an optical signal waveform at a higher peak level.

Then, the pulse signals obtained by photoelectronically converting the optical signal waveform containing the two kinds of information in the photoreceiving device 5 are inputted into the signal-processing section 8 for discriminating between "1" and "0" of the angular signal S₁ and to the signal processing section 9 for discriminating between "1" and "0" of the reference position signal S₂.

In the signal-processing section 8, since the reference voltage V₁ is set lower than the lower peak level P₁, the signal waveform outputted from all of the slits in the code pattern can be shaped into a pulse waveform and detected as the angular signal.

In this case, if the width for each of the slits is equal, the pulse width t_(a) for the angular signal S₁ outputted from the specific slit 2a would be widened (as shown by the chained line in FIG. 3) and, therefore, the pulse width in that portion may be different from that in other portions relative to the rotational angle of the pulse scale, which would possibly lower the accuracy for the detected angle. In the present invention this drawback can be overcome as described below.

In this embodiment, the width of the specific slit 2a is made narrower than that of other slits such that the width of the pulse obtained by shaping the signal waveform outputted from the specific slit 2a based on the reference voltage V₁ for the angular signal S₁ is made equal with the width of the pulse obtained by shaping the signal waveform outputted from the other slits 2, 2. That is, the reference voltage V₁ is set to such a level at which the width of the signal waveform at the higher peak level outputted from the specific slit 2a agrees with the width of the signal waveform at the lower peak level outputted from other slits 2.

Consquently, the angular signals whose waveform is shaped by the reference voltage V₁ are outputted each with an equal pulse width, irrespective of their difference in peak level. In addition, since the angular pitch between the center lines of any two adjacent slits 2 and 2 is made equal, the pulse width of the angular signal S₁ is always equal with respect to the angle of rotation of the pulse scale 1 and the angular signal with the number of pulses corresponding to the angle of rotation is outputted, by which 360° angle for the rotation of the pulse scale 1 can be detected accurately based on the pulse period of the angular signal S₁.

Further, in the signal-processing section 9, since the value for the reference voltage V₂ as the threshold value for the reference position signal S₂ is set between the higher peak level P₂ and the lower peak level P₁ so that only the signal waveform outputted from the specific slit 2a can be detected, only the signal at the higher peak level can be detected as the reference position signal S₂.

Then, the waveform for the angular signal S₁ and the reference position signal S₂ are shaped in the signal-processing sections 8 and 9 based on the reference voltages V₁ and V₂ which are set in accordance with the difference for the peak levels P₁ and P₂, respectively, and are then inputted as pulse signals to the mathematical processing device 10.

The mathematical processing device 10 discriminates two reference position signals "1" inputted successively as the signal indicative of 0° of angle and discriminates the reference position signal "1" inputted by only one as a cylinder-discriminating signal which is outputted on every 60° rotation. The mathematical processing device 10 also detects the angle of rotation of the pulse scale 1 by counting the number of pulses in the angular signal S₁ while referring to the reference position signal S₂ and accumulating the angle of rotation for one pulse period.

In this way, since the angular signal S₁ and the reference position signal S₂ can be extracted from a series of signals, it is only necessary to dispose the and the light-emitting and light-receiving optical fibers 6 and 7 each by one, the number of parts can be reduced to decrease the cost and reduce the size of the connector.

Although the explanation has been made in regard to this embodiment as including a transmission-type optical rotary encoder, the present invention is not restricted only thereto but it is applicable also to a reflection-type optical rotary encoder. In this case, the reflectance in the specific light reflection portion for outputting the reference position signal is made higher, while the reflectance of other light reflection portion except for the specific light reflection portion is made lower, and the circumferential width of the specific light reflection portion is made narrower than that of the other light reflection portion.

For lowering the reflectance, any of optional means can be applied. For instance, the intensity of the reflected light may be weakened by roughening the surface of the light reflection portion, or by narrowing the area of the light reflection portion aligned with the of light projection area.

As has been described above, according to the present invention, since two kinds of information comprising the angular signal and the reference position signal are out-putted from a code pattern formed on a rotatable pulse scale as a series of signals having varying light intensity, it is only necessary to use a pair of a light-emitting device for projecting light to the pattern and a photo-receiving device for detecting optical pulse signals out-putted therefrom, by which the number of parts can be decreased to reduce the cost and facilitate assembling or connection of the optical fibers. In addition, since the number of the devices is decreased, the size of the connection portion between the optical fibers and the control device can be reduced so that the control device can be disposed easily even in a narrow space.

In particular, since an angular signal can always be outputted with the pulse width being equal relative to the angle of rotation, it has an excellent effect of being able to detect the rotational angle with a high accuracy. 

What is claimed is:
 1. An optical rotary encoder, in which both an angular signal and a reference position signal indicative of 0 degrees for an angle of interest are outputted as optical pulse signals in accordance with the angle of rotation of a rotatable pulse scale, and the angle of rotation of the pulse scale is detected based on the angular signal and the reference position signal, wherein the optical rotary encoder comprises:a rotatable pulse scale having a row of code pattern elements formed thereon so as to extend in the circumferential direction thereof and including in said row both a first light-transmitting or -reflecting portion for outputting opticals signals at a first, higher peak level containing both the optical angular signal and an optical reference position signal together, and a second, light-transmitting or -reflecting portion for outputting optical signals at a second, lower peak level containing only the optical angular signal, each said element having a width in said circumferential direction, elements of said first portion being interspersed with elements of said second portion in said row, a signal processing section for discriminating between binary "1" and "0" for electric pulse signals obtained by optoelectronic conversion of said optical signals and waveform shaping of the resultant electric pulse signals based on a first threshold voltage for said angular signal, which is set lower than said lower peak level, and a second threshold voltage for said reference position signal, which is set between said higher peak level and said lower peak level, and the circumferential width of each of said code pattern elements in one of said first and second portions being characteristically narrower than that of each of said code pattern elements in the other of said first and second portions, by a given amount such that the width of each pulse of the electric pulse signal at the higher peak level obtained by the waveform shaping is substantially equal to the width of each pulse of the electric pulse signal at the lower peak level obtained by the waveform shaping, each based on said first threshold voltage, so that electrical pulse signals resulting from sensing elements of said first and second portions are indicative of angular position and electrical pulse signals resulting from sensing elements of said first portion are additionally indicative of a reference position.
 2. An optical rotary encoder in which both an angular signal and a reference position signal indicative of 0 degrees are outputted as optical pulse signals in accordance with the angle of rotation of a pulse scale, and the angle of rotation of the pulse scale is detected based on said angular signal and said reference position signal, wherein the optical rotary encoder comprises:(a) a rotatable pulse scale having a row of code pattern elements formed thereon so as to extend in the circumferential direction thereof and including in said row both a first light light-transmitting or -reflecting portion for outputting optical signals at a first, higher peak level containing both an optical angular signal and an optical reference position signal together, and a second, light-transmitting or -reflecting portion for outputting optical signals at a second, lower peak level containing only the optical angular signal, and (b) a control device including:a light-emitting device for projecting light onto the code pattern and a photoreceiving device for receiving both of two kinds of the optical signals resultingly outputted, which two kinds of optical signals are superposed one upon the other from the code pattern, and converting said two kinds of optical signals into two respective kinds of electric pulsed signals, a pair of threshold voltage generation means, including one threshold voltage generating means for generating a first threshold voltage which is lower than the peak level of said electric pulse signals when only the angular signal is inputted and the other said threshold voltage generating means for generating a second threshold voltage which is higher than the peak level of said electric pulse signals when only the angular signal is inputted and lower than the peak level of said electric pulse signals when both of the angular signal and the reference position signal are inputted simultaneously, and a pair of signal-processing means, one of said signal-processing means being arranged for comparing the electric pulse signals when outputted from the photoreceiving device with the first threshold voltage to discriminate the absence or presence of the angular signal as binary "0" or "1", and the other of said signal-processing means being arranged for comparing the electric pulse signals when outputted from the photoreceiving device with the second threshold voltage to discriminate the absence or presence of the reference position signal as binary "0" or "1", so that the electrical pulse signals outputted from the photoreceiving device are separated into the angular signal and the reference position signal, and wherein the circumferential width of each said first code pattern element is narrower than that of each said second code pattern element in the code pattern of the pulse scale, so that the width of each pulse of said electric pulse signals at the higher peak level is equal to the width of each pulse of said electric pulse signals at the lower peak level, each based on the first threshold voltage.
 3. An optical rotary encoder, comprising:a rotatable disk having provided thereon a single circumferentially extending row of angularly discrete code pattern elements all centered on successively equally angularly spaced radii, but being of two characteristically differing widths, measured circumferentially of the disk, so as to constitute a first set of narrower code pattern elements regularly interspersed in a second set of wider code pattern elements, each code pattern element in said first set being one of: more light transmissive, thicknesswise of said disk, and more light reflective with respect to one face of said disk, relative to each code pattern element in said second set, so that said code pattern elements of said first set, when illuminated, each provide an optical signal which has a first, higher peak level which can be sensed as indicative of a reference position, and said code pattern elements of both said first set and said second set, when illuminated each provide an optical signal which, when sensed at a lower peak level than said higher peak level can be sensed as indicative of angular position; a light-emitting means disposed to successively illuminate individual ones of said elements in said row, and a light-detecting means arranged to successively detect light transmitted through or reflected from individual ones of said elements in said row; a digital signal processing means optoelectronically converting said optical signals to two separate sets of electric pulse signals including a first set which result from optical signals having peak levels which are greater than said lower peak level and thus are indicative of reference position, and a second set which result from optical signals having peak levels which are at least as great as said lower peak level and thus are indicative of angular position; and a mathematical processing unit operatively connected with said digital signal processing means for receiving said two sets of electrical pulse signals and providing therefrom as an output, a continuing and substantially instantaneous detection of the angle of rotation of said rotatable disk relative to at least one reference angular position or said rotatable disk.
 4. The optical rotary encoder of claim 3, wherein:said first and second sets of code pattern elements have characteristically differing widths selected such that at a characteristic voltage level selected for optoelectronically converting said optical signals into said second set of electric pulse signals, all of said electrical pulse signals in said second set are substantially equal to one another in duration. 